Plastic pattern method for investment casting



United States Patent 3,465,808 PLASTIC PATTERN METHOD FOR INVESTMENTCASTING Stuart Z. Uram, Canton, Ohio, assignor to TRW Inc., Cleveland,Ohio, a corporation of Ohio No Drawing. Filed Sept. 7, 1966, Ser. No.577,786

Int. Cl. B22e 9/04; B22c 7/02; B22d 29/00 US. Cl. 16436 2 ClaimsABSTRACT OF THE DISCLOSURE The disclosure relates to a method of forminga shell mold using a pattern composed of a mixture of polystyrene and aWater soluble ethylene oxide polymer and removing the pattern afterformation of the shell mold by treatment with steam under pressure.

The present invention relates to the field of investment casting, andmore specifically to an improved pattern material to be used in thepreparation of ceramic shell molds.

Historically, mold-making procedures using investment techniques startedwith the use of wax as a pattern material giving rise to the name lostWax process for the field of mold manufacture for investment castingusing disposable patterns. In more recent times, there has been anincrease in use of thermoplastic resins, particularly polystyrene, as asubstitute for the was. The plastic patterns have certain advantagesover Wax in some areas. Plastic materials such as polystyrene have goodhot strength properties, dimensional stability, and ease of assembly andhandling. The stiffness of plastic patterns is also desirable inbuilding up the ceramic shell about the pattern.

One of the major drawbacks to the use of such plastic patterns, however,is the necessity to take very special precautions in removing theplastic pattern from the shell without cracking the shell. For example,plastic patterns may tend to distort the green investment more thanwaxes during the meltout operation. The present invention overcomes thisdifliculty by providing a pattern composition which is considerablyeasier to remove from the green investment, using ordinary meltoutprocedures and apparatus, than has heretofore been possible withthermoplastic pattern materials.

One of the objects of the present invention is to provide an improvedpattern-making material of the thermoplastic type which can be readilyremoved without damaging the surrounding green ceramic investment.

Another object of the invention is to provide an improved pattern-makingmaterial of the thermoplastic type capable of producing patterns havinggood surface finish, andv suflicient stiffness and dimensional integrityto provide excellent investment molds.

Another object of the invention is to provide a pattern-making materialemploying polystyrene as the essential pattern-forming material, butbeing readily removed by treatment in a steam autoclave.

A further object of the invention is to provide an improved method forthe manufacture of investment casting molds by employing an improveddisposable pattern material.

A further object of the invention is to provide a method for themanufacture of investment casting molds which makes use of a steamautoclave to eliminate the plastic pattern material from the shellwithout danger of cracking the shell.

In accordance with the present invention, I provide a pattern materialwhich consists essentially of a mixture of polystyrene and a watersoluble polymer compatible with the polystyrene and providing improvedmeltout prop- "ice erties when subjected to high temperatures. In thepreferred embodiment of the present invention, I employ a mixture ofpolystyrene and a water soluble polymer of ethylene oxide, the latterconstituting from 10 to 50% by weight of the mixture of the twoingredients. The term Water soluble as used herein is used to denote amaterial which is soluble in water at least to the extent of providing a5% solution by weight at 25 C.

There are numerous ethylene oxide polymers on the market which aresuitable for combination with polystyrene as a pattern-making material.Particularly good results are obtained through the use of ethylene oxidepolymers which have a molecular weight in the range from about 100,000to about 2,000,000. Such polymers have a viscosity, in a 5% aqueoussolution, of from about to 2000 centipoises at 25 C. In this connection,it should be noted that it is not necessary to use a single ethyleneoxide polymer to provide the desired molecular weight, as a mixture ofpolymers having molecular weights (and consequently viscosities) aboveand below the stated ranges can be used in proper proportions to providean average molecular weight or viscosity in the preferred ranges.

The investment casting mold-making operation using the plastic materialsof the present invention is not changed to any significant degree withthe use of the new pattern material. The mixture of polystyrene and oneor more of the ethylene oxide polymers is placed in a conventionalplastic injection machine and injected into a die, usually at injectionpressures of from 300 to 3500 pounds per square inch. The injectiontemperatures are usually in excess of 250 F. Upon solidification, apattern is produced in the die which has sufficient stiffness anddimensional integrity that it can be used for the manufacture of moldsfor the investment casting process.

The resulting pattern can be used to build up a ceramic shell mold inany of a variety of processes. One particularly preferred method is thatwhich is described in Mellen et al. US. Patent No. 2,932,864 of April19, 1960. In accordance with the procedure described in that patent, thedisposable pattern material is dipped in an aqueous ceramic slurryhaving a temperature about the same as that as the pattern material toform a refractory layer of a few mils in thickness. A typical slurry maycontain ceramic material such as zirconium oxide, a binder such ascolloidal silica, and a thickener and low temperature binder such asmethyl cellulose. The initial layer while still wet is then dusted withsmall particles (40 to 200 mesh) of a refractory glass composition suchas that known as Vycor which is a finely divided high silicon oxideglass containing about 96% silica and a small amount of boric acidtogether with traces of aluminum, sodium, iron and arsenic. The patternwith the dusted wet refractory layer on it can then be suspended on aconveyor and moved through a drying oven having a controlled humidityand temperature, wherein the coated pattern is dried adiabatically.

The steps of dipping, dusting, and adiabatic drying are then repeatedusing air at progressively lower humidities for succeeding coats. Forexample, the first two coats can be dried with air having a relativehumidity of 45 to 55% The third and fourth coats can be dried at arelative humidity of 35 to 45%, the fifth and sixth coats with arelative humidity of 25 to 30%, and finally the last coat with arelative humidity of 15 to 25%.

The first layer is preferably applied at a thickness of about 0.005 to0.020 inch, and the fine refractory particles are dusted onto the wetlayer with sufficient force to embed the particles therein. It ispreferred that the dusting procedure be used to provide a dense uniformcloud of fine particles that strike the wet coating with sufiicientimpact force. The force should not be so great, however, as to break orknock off the wet prime layer from the pattern. This process is repeateduntil a plurality of integrated layers is obtained, the thickness of thelayers each being about 0.005 to 0.020 inch.

After the shell has been completed, the usual shell thickness beingbetween /a and inch, it is thoroughly dried and then the resulting greeninvestment mold is ready for removal of the disposable pattern. With theimproved pattern material of the present invention, pattern removal canbe readily effected by placing the coated pattern in a steam autoclavewhere it is subjected to steam at a pressure of 100 pounds per squareinch or so. This melts the plastic pattern, and the shell may then betransferred to a high temperature firing oven to burn out the last bitof plastic residue and to fire the ceramic material. The resulting shellmold is hard, smooth and relatively permeable.

The following specific examples illustrate the manner in which theimproved pattern compositions of the present invention can be made.

EXAMPLE I A pattern material Was made up with the following composition:

Table I Percent by weight Polyox WSR N-lO (viscosity of -20 centistokesfor 5% aqueous solution at 25 C.) 5 Polyox WSR N3000 (viscosity of22503350 centistokes for 5% aqueous solution at 25 C.) Polystyrene 80The polystyrene employed in the foregoing composition was a commerciallyavailable material of a type suitable for ordinary injection molding. Aceramic shell mold was built up about the resulting pattern, using awater base colloidal silica slurry. After the shell had been thoroughlydried, it was placed in a steam autoclave and pressurized with steam to100 pounds per square inch. The plastic pattern material wassubstantially completely removed, and then the shell was transferred toa high temperature firing oven operating at a temperature of 1500 to1900 F., to complete the rigidification of the mold structure. The shellmold that resulted was smooth, strong and relatively porous.

The same procedure was carried out using polystyrene alone at thepattern material. It was found that under the conditions of operation,the shell cracked in the autoclave when pattern removal was attempted.

The properties of the pattern material can be controlled by adjustmentof the relative amount and character of the ethylene oxide polymeremployed. Using polymers which have very low molecular weight andviscosity produces mixtures which are more difiicult to inject, but thepatterns produced have the most suitable surface finish. Adding up to ofa high viscosity, high molecular material improves the injectionproperties but reduces the quality of the surface finish. As the amountof ethylene oxide polymers increases, the stiffness of the patternstends to be decreased, but at the same time, it considerably simplifiesthe elimination of the plastic pattern from the finished ceramic shell.

From the foregoing, it will be understood that the pattern makingcomposition of the present invention provides a plastic pattern materialwhich has all of the advantages of plastic patterns while making it fareasier to remove the pattern material from the green investment mold.The pattern materials of the present invention can be used withconventional mold-making processes and with conventional equipment sothat the cost of the investment mold-making process is not significantlyincreased.

It should be evident that various modifications can be made to thedescribed embodiment without departing from the scope of the presentinvention.

I claim as my invention:

1. The method of forming an investment mold which comprises forming amixture of polystyrene and a Water soluble ethylene oxide polymercompatible with said polystyrene, with said polymer of ethylene oxideconstituting from 10 to 50% weight of the mixture, shaping said mixtureinto a pattern, investing the resulting pattern in a particulate ceramicinvestment material to form a ceramic shell mold and treating theresulting shell mold with the steam under pressure to remove the patternmaterial from said shell mold.

2. The method of claim 1 in which said polymer of ethylene oxide has amolecular weight in the range from about 100,000 to 2,000,000.

References Cited UNITED STATES PATENTS 2,420,851 5/ 1947 Zahn et al.2,815,552 12/1957 Turnbull et al. 2,456,424 12/1948 Neiman 164-2462,994,931 8/1961 Operhall et al. 164246 3,063,113 11/1962 Operhall etal.

FOREIGN PATENTS 1,215,874 5/1966 Germany.

I. SPENCER OVERHOLSER, Primary Examiner V. K. RISING, Assistant ExaminerUS. Cl. X.R. 164-132, 246

